Optical device and method for the non-intrusive measuring of the temperature of a flowing liquid

ABSTRACT

The invention relates to an optical method and to an optical device based on a laser source for implementing the optical method for nonintrusively measuring the temperature of a flowing liquid by using the fluorescence induced by a laser beam in a measurement volume ( 3 ) of the liquid, the process consisting in using a single temperature-sensitive fluorescent tracer and at least two separate spectral detection windows on this same tracer, after molecular dilution of said tracer in said liquid medium.

[0001] The present invention relates to an optical method and to an optical device based on a laser source for nonintrusively measuring the temperature in a flowing liquid.

[0002] Measurement devices based on a laser source are already used, for example that of application EP 345188 based on the observation of interference fringes created within a flowing fluid in a pipe, at the output of two optical fibers transporting the light from an external laser source.

[0003] This apparatus is satisfactory for this type of situation, but it does require intrusion in the fluid studied, something that is not always possible.

[0004] Various nonintrusive techniques for measuring the parameters of a fluid (for example, temperature or pressure) are already known. For example, patent application FR 2 579 320 discloses an ultrasonic wave measurement method intended for the thermal monitoring of nuclear reactors. This technique allows only a single parameter of the fluid to be measured, namely its temperature.

[0005] The main drawback of the nonintrusive temperature measurements known at the present time is their excessively long response time.

[0006] In difficult situations such as:

[0007] measurement of the temperature of droplets (in a spray for example), the measured volume of which changes owing to evaporation over the time of measurement;

[0008] liquid-gas two-phase flow;

[0009] measurement of the temperature field in a plane;

[0010] measurement of the temperature in a turbulent flow;

[0011] said measurement remains difficult, if not impossible to carry out, and there are difficulties in obtaining an apparatus that has a short response time and disturbs the flow as little as possible.

[0012] To solve these difficulties, the inventors have resorted to optical methods, and in particular to a laser-induced fluorescence method already studied in the laboratory by the inventors for determining the concentration of a fluid, the principle of which is recalled below:

[0013] Fluorescence, a physical phenomenon known for a long time, is the consequence of an excited state of a fluorescent species being deactivated toward a ground state by spontaneous emission. The excited state may be induced by laser radiation whose wavelength coincides with the absorption spectrum of the fluorescent species. The time between the absorption and emission of a photon is of the order of a few nanoseconds, which makes the technique applicable to the study of rapidly varying phenomena. The time resolution may reach a few tens of kHz. When laser radiation passes through a medium seeded with a low concentration (or a higher, but constant or slightly varying concentration) of a fluorescent tracer, the fluorescence intensity may be expressed by the equation:

I _(fluo) =K _(opt) K _(spec) CV _(c) I _(o) e ^(β) ^(₁) ^(/T)

[0014] where K_(opt), V_(c), I_(o) and C are the coefficient characterizing the optical system, the volume in which the fluorescent photons are collected, the incident laser intensity and the molecular concentration of the fluorescent tracer, respectively. K_(spec) and β are constants depending only on the characteristics of the molecule used as fluorescent tracer.

[0015] Applying this principle allows the temperature to be obtained in simple situations in which the volume, the concentration and the local laser intensity are well controlled or remain constant.

[0016] A first technique applying this physical principle to the measurement of temperatures partly eliminates the drawbacks mentioned—it consists in using two fluorescent tracers—and solves the problem of the laser intensity and of the volume, but many difficulties nevertheless remain:

[0017] the concentration of each of the two tracers must be controlled;

[0018] the emission spectra of the two tracers must be sufficiently separate so as to be able to separate the fluorescent emissions of each of the tracers by means of a set of interference filters, but this is difficult to achieve in practice.

[0019] To resolve all of the above problems, the inventors had the novel idea of using a single tracer with molecular dilution in said liquid medium whose temperature it is desired to measure.

[0020] Moreover, in the known prior art there is U.S. Pat. No. 5,788,374 A. This relates in fact to chemiluminescence induced by white light (lamp or xenon).

[0021] The photophysics used is completely different from that of the invention and in fact relies in reality on the detection of fluorescence in two spectral bands but from two different products created by a chemical reaction, the kinetics of which depend on the temperature. This reaction is of the type: M+hν→M*. In the presence of a partner N (which in the patent cited seems to be the matrix of the molten polymer in which the fluorescent product is in solution), the monomer in the excited state M* may either emit fluorescence in a wavelength range 1M*→M+hν₁ or be converted into an exciplex E* after reacting with the partner N, which itself will give off fluorescence in a wavelength range 2, according to the reaction:

M*+N→E*

E*→E+hν ₁

[0022] The ratio of the fluorescence of the monomer (wavelength range 1) to that of the exciplex (wavelength range 2), which depends on the temperature, is therefore detected.

[0023] In that technique, there are therefore two fluorescent products, whereas in the two-color fluorescence technique according to the invention there is only a single fluorescent product.

[0024] In addition, the two-color fluorescence method according to the invention uses ultrarapid photophysics principles relating to electronic transition and collisional deactivation phenomena and not the kinetics of a chemical reaction induced by irradiation. The response time of the technique is thereby considerably improved.

[0025] Also known is French patent application FR 2 484 639. The method described in that document is an intrusive method since it requires the presence of sensors at various measurement locations, whereas the method according to the invention is a nonintrusive method.

[0026] Furthermore, the fluorescent material used is a solid-state material (page 8, lines 2-3), whereas the tracer according to the invention is a tracer in molecular solution in the liquid whose temperature is measured.

[0027] Also known is Patent Abstract of Japan; according to the summary provided, the method consists in seeding with fluorescent particles and not with a tracer in molecular solution, and it uses irradiation with UV light and not a laser radiation.

[0028] The invention consists of an optical method for nonintrusively measuring the temperature of a flowing liquid, characterized by the use of the fluorescence induced by laser radiation in a measurement volume of the liquid and characterized in that it consists in using a single temperature-sensitive fluorescent tracer and at least two separate spectral detection windows on this same tracer, after molecular dilution of said tracer in said liquid medium.

[0029] The method preferably comprises the following steps:

[0030] reception of the optical signal and complete elimination of scattering or reflection of the exciting laser component;

[0031] splitting of the optical signal into several light signals;

[0032] creation of a detection window for each light signal obtained after the above splitting;

[0033] amplification of said light signals received in the detection windows and conversion of said light signals into as many electrical signals; and

[0034] acquisition, processing and display of the above electrical signals.

[0035] The invention also consists of a device for nonintrusively measuring the temperature of a flowing liquid by using the fluorescence induced by laser radiation in a measurement volume of the liquid, characterized in that it is designed for implementing the method according to the invention and in that it mainly comprises:

[0036] a single reception channel with:

[0037] a holographic band-rejection filter

[0038] a unit for splitting the optical signal into two light signals;

[0039] in each optical measurement channel:

[0040] a filter for creating a measurement window

[0041] an amplifier for amplifying and converting the light signals into electrical signals;

[0042] a computing system.

[0043] Preferably, the laser radiation is chosen from the collection: single laser beam, double laser beam, laser sheet.

[0044] Preferably, the filter creating the measurement window may be chosen from the collection: band-pass interference filter, high-pass filter, low-pass filter.

[0045] Preferably, a band-pass interference filter of bandwidth Δλ₁, through which the first signal passes, centered on a wavelength λ₁, is chosen for one measurement channel and a high-pass filter through which the second signal passes, letting through the optical intensity above a threshold wavelength λ₂, is chosen for the other measurement channel.

[0046] A better understanding of the invention will be gained from the description of a nonlimiting embodiment which follows, with reference to the following appended figures:

[0047]FIG. 1: a diagram showing the principle of a device according to the invention;

[0048]FIG. 2: a graph showing the positions of the detection windows on the emission spectrum of Rhodamine B.

[0049] The original method of the invention consists in using a single fluorescent tracer. Said tracer is in molecular dilution in said liquid medium whose temperature it is desired to measure, there is no transformation of the chemical type and its fluorescent properties arise from one and the same molecule.

[0050] In the nonlimiting example described below, the tracer used is Rhodamine B (C₂₈H₃₁ClN₂O₃). This is known to be particularly sensitive to temperature. In addition, the temperature sensitivity of this tracer is different depending on the spectral band of the fluorescent spectrum in question. The invention proposes, according to a first example, to use a single tracer and two separate spectral detection windows on this same tracer.

[0051] The resulting sensitivity of the method as used in the invention is around 2% of the variation in the fluorescence signal per ° C., which, taking into account the observed signal-to-noise ratio, leads to an accuracy to within 1° C. in the temperature.

[0052] Detection therefore takes place on two predefined spectral windows and not by studying complete spectra, thereby not requiring the use of a spectrometer, but only of simple photodetectors (photodiodes, photomultipliers, CCD cameras, etc.).

[0053] To implement this method, the inventors have produced a device (1) based on the fluorescence induced by laser radiation (2) in a measurement volume (3) of a flowing liquid, allowing splitting of the signals and detection thereof. The laser radiation may be in the form of one or two laser beams, or else a laser sheet. In particular the use of two laser beams intersecting at a point defining the measurement volume makes it possible at the same time to measure the velocity by means of a commercially available velocity meter system. The device comprises, mainly, but not limitingly:

[0054] A single reception channel with:

[0055] a holographic band-rejection filter (6) for removing all scattering or reflection of the exciting laser component (12);

[0056] a unit (4) for splitting the optical signal (5) into at least two equal or non-equal light signals (5 a, 5 b), for example two light signals, by means of two neutral splitters (4 a, 4 b) to be defined according to the desired power in each channel;

[0057] In each optical measurement channel:

[0058] a filter, possibly a band-pass interference filter, a high-pass filter or a low-pass filter, for creating a measurement window appropriate to the tracer used. For the example described here (Rhodamine B dissolved in ethanol), the following were preferably chosen:

[0059] a band-pass interference filter (7) through which the signal (5 a) passes, making it possible to obtain the first measurement window (10) visible in the graph of FIG. 2. For the tracer used, the filter (7) is centered on a wavelength λ₁±Δλ₁, preferably 530 nm±5 nm;

[0060] a high-pass filter (8) through which the second signal (5 b) passes, letting through the optical intensity above a threshold wavelength λ₂, preferably λ₂=approximately 590 nm;

[0061] the essential criterion in the choice of wavelengths is that of having different temperature sensitivities. The windows and the wavelengths are chosen according to the temperature response curve of the tracer or of the tracer-solvent mixture. Cost criteria may also be involved in the choice of filters. In the entire description and the claims, the term “window” is understood in its general meaning, a window possibly being composed of one or more wavelength ranges, which may or may not be limited, likewise the term “separate” does not exclude any partial overlap or the inclusion of the windows, and the expressions “measurement window” and “detection window” have the same meaning;

[0062] An amplifier (9 a, 9 b) for amplifying and converting the light signals (5 a, 5 b)into electrical signals, for example a photomultiplier, a photodiode or a CCD sensor. These amplifiers provide the precise measurement, with a short response time, of the fluorescence intensities.

[0063] A computing system (11) with its software:

[0064] This receives the electrical signals via an acquisition system (not shown), processes them and displays the results on a screen.

[0065] The collected fluorescence intensity of the first signal (5 a) can be written according to the simplified equation established by the inventors:

I _(f1) =K _(opt1) K _(spect1) CI _(o) V _(c) e ^(β) ^(₁) ^(/T)

[0066] Likewise, the fluorescence intensity of the second signal (5 b) satisfies the equation:

I _(f2) =K _(opt2) K _(spect2) CI _(o) V _(c) e ^(β) ^(₂) ^(/T)

[0067] Both the above equations are expressed with the following proviso:

[0068] the relevant absorption coefficients with respect to the fluorescence intensity are similar in the relevant spectral windows.

[0069] Or else:

[0070] the product C.x may vary slightly in relation to the control, thereby allowing simplification (x denoting the optical path traveled by the fluorescence signal in the absorbent medium).

[0071] Thus, the ratio of the measurement of the fluorescence intensity of the signals received in the detection windows is characterized by the equation: $\begin{matrix} {R_{f} = {\frac{I_{f1}}{I_{f2}} = {Ke}^{{({\beta_{1} - \beta_{2}})}/T}}} & \left( E_{1} \right) \end{matrix}$

[0072] where K is a constant that only depends on the optical system used and on the spectroscopic properties of the molecule used as fluorescent tracer. This ratio has the advantage of being independent of the concentration of fluorescent tracer, of the exciting laser intensity and of the excited fluorescent volume. The apparatus constant (K) is determined by a single initial calibration point at a known temperature T_(o). Equation (E₁) is thus transformed to: $\begin{matrix} {{\ln \quad \left( \frac{R_{f}}{R_{fo}} \right)} = {\left( {\beta_{1} - \beta_{2}} \right)\quad \left( {\frac{1}{T} - \frac{1}{T_{o}}} \right)}} & \left( E_{2} \right) \end{matrix}$

[0073] where R_(fo) is the ratio at the temperature T_(o). This law was confirmed experimentally.

[0074] In addition to the temperature, it is possible at the same time to obtain the concentration of the liquid, or the measurement volume (3). When the relevant absorption coefficients with respect to the fluorescence intensity are not similar in the relevant spectral windows, or else the product C.x varies significantly with respect to the control, the absorption phenomena of the fluorescence must be taken into account. In this case, a third channel and a third measurement window may be added so as to quantify this absorption.

[0075] The tracer chosen was Rhodamine B diluted in alcohol to 2 mg/l, this being satisfactory and allowing two spectral bands to be isolated, but other tracers, such as for example Oregon 488, Rhodamine 6 G, and other dilution liquids such as water, could be used.

[0076] The invention may advantageously be applied for measuring:

[0077] the temperature of spray droplets. The difficulty as regards the prior art resides in the variation in the measurement volume because of the temperature. Since this parameter is absent from equation E₂, it has no influence on the measurements and the invention can be used;

[0078] the temperature in a turbulent flow, for which apparatuses with a suitable response time are unknown; and

[0079] the temperature distribution in a measurement plane through which a flowing fluid passes, independently of the variations in the local laser intensity. 

1. An optical method for nonintrusively measuring the temperature of a flowing liquid, characterized by the use of the fluorescence induced by laser radiation in a measurement volume (3) of the liquid and characterized in that it consists in using a single temperature-sensitive fluorescent tracer and at least two separate spectral detection windows on this same tracer, after molecular dilution of said tracer in said liquid medium.
 2. The method as claimed in claim 1, characterized in that the tracer used is Rhodamine B (C₂₈H₃₁ClN₂O₃).
 3. The method as claimed in either of claims 1 and 2, characterized in that it comprises at least the following steps: reception of the optical signal and complete elimination of scattering or reflection of the exciting laser component; splitting of the optical signal into several light signals; creation of a detection window for each light signal obtained after the above splitting; amplification of said light signals received in the detection windows and conversion of said light signals into as many electrical signals; and acquisition, processing and display of the above electrical signals.
 4. The method as claimed in claim 3, characterized in that it uses two detection windows and in that the ratio of the measurement of the fluorescence intensity of two signals received in the two detection windows is characterized by the equation (E₁): $R_{f} = {\frac{I_{f1}}{I_{f2}} = {{Ke}^{{({\beta_{1} - \beta_{2}})}/T}.}}$


5. The method as claimed in claim 4, characterized in that the apparatus constant (K) is determined by a single initial calibration point at a known temperature T_(o), equation (E₁) being thus transformed to: $\begin{matrix} {{\ln \quad \left( \frac{R_{f}}{R_{fo}} \right)} = {\left( {\beta_{1} - \beta_{2}} \right)\quad \left( {\frac{1}{T} - \frac{1}{T_{o}}} \right)}} & \left( E_{2} \right) \end{matrix}$

where R_(fo) is the ratio at the temperature T_(o).
 6. The method as claimed in one of claims 1 to 5, characterized in that it furthermore allows at the same time at least one of the following characteristics to be measured: velocity of the liquid, concentration of the liquid, measurement volume.
 7. A device for nonintrusively measuring the temperature of a flowing liquid by using the fluorescence induced by laser radiation in a measurement volume (3) of the liquid, characterized in that it is designed for implementing the method according to one of claims 1 to 6 and in that it mainly comprises: a single reception channel with: a holographic (6) band-rejection filter (6) a unit (4) for splitting the optical signal into two light signals (5 a-5 b); in each optical measurement channel: a filter for creating a measurement window an amplifier (9 a-9 b) for amplifying and converting the light signals (5 a-5 b) into electrical signals; a computing system (11).
 8. The device as claimed in claim 7, characterized in that the filter creating the measurement window may be chosen from the collection: band-pass interference filter, high-pass filter, low-pass filter.
 9. The device as claimed in claim 8, characterized in that a band-pass interference filter (7) of bandwidth Δλ₁, through which the first signal (5 a) passes, centered on a wavelength λ₁, is chosen for one measurement channel and a high-pass filter (8) through which the second signal (5 b) passes, letting through the optical intensity above a threshold wavelength λ₂, is chosen for the other measurement channel.
 10. The device as claimed in claim 9, characterized in that λ₁ is equal to approximately 530 nm.
 11. The device as claimed in one of claims 7 to 10, characterized in that λ₂ is equal to 590 nm approximately.
 12. The device as claimed in one of claims 9 to 11, characterized in that a third measurement window and a third channel may be added in order to quantify the absorption phenomena of the fluorescence.
 13. The device as claimed in one of claims 7 to 12, characterized in that the laser radiation is chosen from the collection: single laser beam, double laser beam, laser sheet.
 14. The device as claimed in one of claims 7 to 13, characterized in that the detection is performed by simple photodetectors. 